The invention relates to a low-pass filtering device for a private installation connected to an access network carrying narrow-band services (analogue or ISDN) and broad-band services (xDSL or HomePNA) and the private installations including such devices.
1. Field of the Invention
Private installation means a Customer Premises Network (CPN). It starts at the terminal strip of the premises (NID, Network Interface Demarcation), and comprises all the cables, copper pairs and telephone sockets. It is connected (possibly via active equipment such as a PABX) to a network for access to narrow-band services and to broad-band services. It will in practice be a telephonic and teledata installation connected to the telephone network comprising one or more narrow-band terminals such as the telephone handsets, faxes, answering machines, modems or any other type of analogue or digital equipment (ISDN) and one or more broad-band terminals based on DSL or HomePNA technology.
Hereinafter the narrow-band terminals will be referred to by the abbreviation NBT and broad-band terminals by the abbreviation BBT.
Broad-band service means any service transmitted in a spectral band situated above narrow-band services (that is to say telephony or ISDN, frequencies higher than 10 kHz). Typically these are services delivered by the xDSL network, or based on purely private systems known as HomePNA, for example.
The term xDSL will be given to the broad-band services grouping together all the technological families such as SDSL, ADSL and ADSL-lite.
HomePNA is a Consortium set up in 1998 with a view to federating the specifications of transmission systems on telephone pairs: Home Phoneline Networking Alliance. By extension of this description, the so-called HomePNA systems will designate all transmission systems on a telephone copper pair, whether or not in conformity with the specifications of the HomePNA Consortium.
2. Description of the Related Art
The publications produced up to the present time relate exclusively to the following subjects:                ADSL-lite and more generally the problems of distributed filtering and its impact on telephony;        design of low-pass filters.        
The publication researches emanating from the following bodies:                UAWG: Universal ADSL Working Group        ADSL-Forum        ITU SG15Q4 International Telecommunications Union-Study Group 15-Question 4        
Amongst the recommendations and standards relating to the technical field of the invention, there are the following:                ITU-T:        SG15/Q4: determination G.992.1 (ADSL modems),        SG15/Q4: determination G.992 (ADSL-lite or Splitterless modems),        SG15/Q4: determination G.996.1 (test procedures for evaluating performances of DSL systems),        Recommendation Q.552 “Transmission characteristics at 2-wire analogue interfaces of digital exchanges”.        
And of the                ETSI:        TBR21: Terminal equipment (TE); Attachment requirements for pan-European approval for connection to the analogue Public Switched Telephone Networks (PSTNs) of TE (excluding TE supporting the voice telephony service) in which network addressing, if provided, is by means of Dual Tone Multi Frequency (DTMF) signalling,        TBR37: Draft EN 301 437 V1.1.1 (1998-09)—Terminal Equipment (TE); Attachment requirements for pan-European approval for connection to the analogue Public Switched Telephone Networks (PSTNs) of TE supporting the voice telephony service in which network addressing, if provided, is by means of Dual Tone Multi Frequency (DTMF) signalling.        
The situation with the problem encountered by the applicant is as follows:
An IPC is connected to an access network. It makes it possible to deliver narrow-band services to the different NBTs.
This same IPC can be used for delivering broad-band services to one or more BBTs (either of the xDSL type, or a private LAN of the HomePNA type for example).
The simultaneous transportation of the two ranges of services on the same IPC is effected by a frequency multiplexing of the corresponding signals.
Frequency multiplexing of broad-band and narrow-band services on the same IPC is liable to require the use of a low-pass filter (or LPF in English terminology) in front of (or in) one or more NBTs in order to guarantee transparency between the two ranges of services (no mutual interference). The concept of distributed filters (‘distributed LPF’, or more generally ‘distributed filter’) is then evoked. This concept is detailed below:
The use of a number of filters which is a prior unknown and may change over time then poses problems of impedance mismatching between the TBE or NBTs and the private installation. For conventional telephony services, for example, this impedance mismatching results in an impairment of the phonic quality.
Bringing an ADSL modem into service conventionally requires the installation of a separator or splitter for narrow-band signals (analogue or ISDN services) and ADSL signals (broad-band services) at the input to the private installation. Each of the services is then routed separately over two distinct infrastructures (copper pairs) to the ad-hoc NBTs or BBTs (typically an ADSL modem).
During 1998 a new concept arose: “splitterless”, which assumes an elimination of the signal separator. However, in the great majority of cases, it then becomes necessary to install low-pass filters before some or all of the NBTs, failing which the broad-band signals interfere with the telephony and vice-versa.
The concept of distributed filtering or “Distributed Filter” is then evoked. The essential aspect of this concept lies in the possibility for the client of installing the ADSL modem and the filters in an autonomous and simple fashion as illustrated by the diagram in FIG. 1A.
An example of a filter used in the prior art is illustrated in FIG. 1B. This is a filter sold by the company Excelsus.
In order to be viable such a solution requires compliance with the following points:                1. the use of distributed filters must preserve the quality of narrow-band service perceived by each NBT;        2. the use of narrow-band services must not interfere with the broad-band services carried by the ADSL signals (for example hanging up or picking up a telephone set), and vice-versa;        3. points 1 and 2 must be complied with whatever the number of filters installed in the private installation of the customer;        4. the type of filter used must be independent of the number of filters installed in the private installation of the customer;        5. the unit price of each filter must remain low.        
When a filter is inserted in the private installation of the customer, it must not impair the retrieval of the narrow-band service. It must for example have an impedance matched to the access impedance of the NBT. An impedance mismatch results in an impairment of the parameters of retrieval of the analogue service, and therefore non-conformity with the specifications or recommendations current in the country in question.
In terms of voice services, for example, an impedance mismatch between an NBT and the telephone access causes a phenomenon of local echo, whose level is related to the degree of mismatching.
In the European recommendations and standards, for example, the NBTs (voice and non-voice) can have several types of impedance described in the ETSI publication: -TBR21 and TBR37.
At the international level, the recommendations and standards published by the ITU-T deal with the same points: recommendation Q.552 “Transmission characteristics at 2-wire analogue interfaces of digital exchanges”.
These publications also describe the parameters for qualifying the narrow-band services, and give recommendations to be complied with in terms of “Return Loss”, or insertion losses, and insertion loss distortion, etc.
Putting several filters in parallel in front of as many NBTs modifies the resulting impedance of the assembly formed by the private infrastructure of the customer (IPC) and the filters: “IPC+filters” seen at each of the NBTs.
The only exception to this rule is the so-called first order filters. This is a case of simple inductors. Putting several of them in parallel does not modify the resulting impedance at the IPC.
The drawback is as follows: the stiffness of a filter is as a first approximation n×6 dB per octave, where “n” is the order of the filter. The first order filters do not sufficiently protect the narrow-band services from the broad-band ones. In terms of telephony, a residual noise (hiss) is then perceptible.
For filters with an order greater than 1, one possible solution would consist of designing them so that the impedance seen by each NBT is matched to a given configuration. For example, if the installation of the customer requires the use of “n” filters, it is possible to manufacture a low-pass filter adapted to this particular situation: “n” filters installed in parallel on the IPC.
The drawback is then: the addition or removal of a filter changes the impedance of the whole, and leads to the situation described above: mismatched impedance between NBT and the assembly consisting of IPC and filters.
A new class of filter has also been proposed by B. Beeman-Siemens Telecom Networks. The objective sought for consists, for this technique, of modifying the impedance Z of the filter in order to limit the variation in the “Return Loss” level (measured here at 4 kHz) when the number of filters installed in parallel changes.
Each filter does not individually have an impedance strictly matched to the NBT, but the return loss varies little within the limit of a few given configurations.
This solution reduces the effect of the number of filters, but does not cancel it out completely.
It is not satisfactory since it does not guarantee compliance with the current recommendations in the different countries (for example ITU-T q; 552, or ETSI TBR21 and TBR37).
In conclusion, there does not exist at the present time any device for simultaneously reconciling the following aspects:                Preservation of the quality of the analogue or ISDN service or services,        Design of a single model of filter able to be installed before (or in) one or more NBTs, the total number of filters being indeterminate and variable over time.        